Isotactic polypropyrene has inferior impact resistance in spite of the superior rigidity and heat resistance. To improve the impact resistance of polypropylene while maintaining the superior rigidity, various technologies for producing resin compositions by combining ethylene-propylene rubber with crystalline polypropylene by polymer blend have been developed. However, producing a resin composition exhibiting well-balanced rigidity and impact resistance has been difficult because it is difficult to highly disperse different polymers at the micron level using a polymer blend technique. Polymer blend requires a very expensive process of kneading different types of polymers, which sometimes increases the polypropylene production cost at least twice that of conventional propylene polymers.
As a means for improving the problem in the polymer blend technique, a chemical blending method of polymerizing stepwise propylene and ethylene, and optionally other olefins, to produce a block copolymer has been studied. In a common chemical blending method, a block copolymer is manufactured in two or more steps, wherein propylene is first polymerized, then ethylene, propylene, and other olefins are copolymerized. In this instance, the proportion of rubbery polymer produced by the copolymerization of ethylene and propylene is increased in some processes to improve the impact resistance. However, the produced rubbery components may deposit on the surface of polymer particles, giving rise to adhesion among the polymer particles and adhesion of the polymers to the inner wall of the production apparatus. Because of this, it is difficult to produce a block copolymer for a long period of time in a stable manner.
To solve this problem Japanese Patent Application Laid-open No. 3-62805 discloses a solid catalyst component for polymerization of olefins having pores with a certain pore size. The solid catalyst component is produced by preparing a magnesium chloride-alcohol addition product, dealcholating the addition product with heating to produce porous magnesium carrier particles, and treating the porous magnesium carrier particles with a titanium compound.
A propylene block copolymer consists of a propylene polymer section (matrix) and an ethylene-propylene copolymer dispersed in the matrix. The propylene block copolymer produced using the above-mentioned conventional catalyst contains very large ethylene-propylene copolymer particles (or the polymer section) because of the large pore size in the solid catalyst component particles. Because of this, the ethylene-propylene copolymer particles may deposit on the surface of polymer particles. In particular, ethylene-propylene copolymer block particles continue to grow after the copolymer has been crystallized, making it difficult to obtain a block copolymer in which minute rubber components are sufficiently dispersed. For this reason, the impact resistance cannot be improved in spite of an increase in the proportion of rubber components. In this manner, no conventional catalysts could produce a propylene block copolymer with greatly improved impact resistance.
Therefore, an object of the present invention is to provide a solid catalyst component for polymerization of olefins and a catalyst containing the solid catalyst component that can produce a propylene block copolymer with a novel structure, in which rubber components (or ethylene-propylene copolymer) in the propylene polymers are well dispersed when the rubber components are produced at a higher ratio in the block copolymerization of propylene and ethylene, resulting in the propylene block copolymer containing polymer particles exhibiting only very slight adhesion among themselves and superior impact resistance, as well as the propylene block copolymer produced using such a catalyst.